In the field of magnetic recording devices, data such as computer programs, databases, spreadsheets, e.g. are usually stored onto a magnetic medium as a series of binary bits. Typically, the magnetic medium takes the form of a circular disk which is rotated about a spindle. A transducer, also known as a "head", is used to write bits of data onto the spinning circular disk. Sometimes the same head is also used to read the bits of data off the spinning disk. In other instances, a separate head is used in the reading process in order to realize greater areal density for storage.
One such type of heads used only for reading data, is referred to as a Magnetoresistive Sensor (MR Sensor). MR sensors comprise a segment of soft-magnetic material whose electrical resistance varies when subjected to a varying magnetic field. This effect is used in Magnetoresistive Recording Heads (MR Heads) to sense the information recorded along tracks of a magnetic recording medium.
FIG. 1 illustrates the principle of operation of an MR sensor whose active part comprises a rectangular piece of magnetoresistive material 101 with a longitudinal axis, "a", parallel to the recording medium and a transverse axis, "b", which is perpendicular to the recording medium. A sense current, "I", flows along the longitudinal axis of the sensor having a resistance R.sub.0. The electrical voltage across the sensor varies in a well known fashion with the orientation, .theta., of its magnetization, M, as V=I. R.sub.0 (1+c.sub.mr) cos.sup.2 .theta., where c.sub.mr is the magnetoresistive coefficient of the sensor material and .theta. is the angle between the current, I, and the magnetization, M. Magnetic fields, H.sub.S, from the recording medium produce variations of .theta. that are linear relative to, H.sub.S, when the quiescent orientation of the magnetization, .theta..sub.0 is roughly at a 45.degree. angle relative to the current. This is achieved by injecting suitable amounts of magnetic biasing flux into the sensing segment, such as to provide for flux continuity along both axis.
This necessitates a biasing flux M.sub.t =M sin .theta..sub.0 along the transverse axis and a biasing flux M.sub.1 =M cos .theta..sub.0 along the longitudinal axis. Hence, a complete sensor embodiment includes transverse and longitudinal biasing means in addition to the sensing segment. The need for transverse biasing provisions has been recognized early, and several biasing schemes have been disclosed in the prior art. They typically employ a laminate of magnetic and nonmagnetic films such that the sensing current, I, generates a transverse magnetic biasing flux within the magnetic laminate circuit.
For instance, the well known Soft-Film Biasing scheme shown in FIG. 2(a) comprises an MR film segment (MR) 201 and a Soft-Film Biasing segment (SB) 202 in a plane-parallel position with each other. The two films are separated by a Spacer segment (SP). The spacer may be quite thin (e.g., 200 A), as it only serves to break the magnetic "exchange coupling" between the MR 201 and the SB 202 film. The sense current, I, flowing through the less resistive (MR) segment 201 saturates the (SB) segment 202 of the appropriate thickness in a perpendicular direction to produce the required biasing flux, M.sub.t.
Mutual biasing schemes like the one shown in FIG. 2(b), employ two identical MR films (MR1) 203 and (MR2) 204 in a plane-parallel position. Here, the sense current is equally divided between the two films 203-204 such that the films mutually bias each other, with the magnetization M1 and M2 rotated in opposite directions. This embodiment is designed to operate in a differential mode with the output signal proportional to the difference in field seen by the two MR segments.
The main difference between the single MR element device of FIG. 2(a) and the dual element device of FIG. 2(b) is that the spatial resolution capability of the former relies on the presence of two magnetic shields (not shown) enclosing the sensor and is governed by the spacing between the two shields. In contrast, the spatial resolution capability of the differential device is inherent and governed by the spacing between the two MR elements.
These and other transverse biasing schemes are well known and practiced in the prior art. The need for longitudinal biasing had also been recognized, but its practical realization proved to be more difficult. This is because in contrast to transverse biasing, one cannot readily utilize the sense current to activate a longitudinal biasing circuit. Consequently, the disclosed longitudinal biasing schemes employ some type of permanent magnet configuration in thin film form for longitudinal biasing. These embodiments require elaborate fabrication steps and are afflicted with a variety of problems and limitations.
For example, U.S. Pat. No. 4,639,806 issued to Kira and U.S. Pat. No. 4,663,685 issued to Tsang, recognized the need for having longitudinal biasing means attached to the MR sensing segment. Kira and Tsang constructed such means in similar fashion, namely by superpositioning a hard-magnetic film onto the end regions of the MR sensor. These end-segments then become inactive because of the presence of the hard-magnetic film and because of the presence of an additional conducting film over the same area. The two inventions differ in that Kira's hard magnetic film is ferromagnetic whereas Tsang's hard-magnetic film is antiferromagnetic (producing no external magnetic flux). In both cases, these longitudinal biasing means accomplish the "freeze" of the magnetization within these end-regions.
In an additional process step, a large external magnetic field is used to magnetize the end-segments along the longitudinal direction and thereby create longitudinal biasing flux. In Tsang's invention, the biasing flux equals the magnetization of the MR film. Kira's method injects an additional amount of flux that equals the magnetization of his ferromagnetic biasing film. Having too much biasing flux is undesirable as it renders the sensitivity within the sensor segment non-uniform, quenching it toward the attached end-segments. Some adjustment is possible, however, by magnetizing the end-segments at some canted angle relative to the longitudinal direction.
The main problem with both inventions is that their longitudinal biasing means do not control the magnetization within the soft-magnetic biasing film. This causes unstable operation and pick-up of extraneous magnetic signals from the end-regions. These prior art embodiments are also quite nonplanar, which produces a loss of spatial resolution capability.
These flaws are corrected in U.S. Pat. No. 4,713,708 by controlling the magnetization of both the MR film and the SB film underneath the end-regions. This is achieved at the expense of added fabrication complexity. Additionally, there is the problem of accurate alignment between the different layers and the preclusion of in-situ deposition.
U.S. Pat. No. 4,771,349 seeks to allow for in-situ deposition of all films comprising the sensing segment. This is achieved by subsequently removing, under the end-regions, the SB and SP layers and then superpositioning the longitudinal biasing means over the underlying MR layer only. The main problem with this approach is fabrication complexity and the creation of partial heterogeneous junctions within the current path.
The objective of U.S. Pat. No. 5,018,037 is to simplify fabrication requirements by constructing contiguous sensing and biasing segments without any layer in-common. A problem with this invention is the creation of a heterogeneous junction in the current path.
U.S. Pat. No. 5,005,096 utilizes a hard-magnetic film to cause "magnetostatic biasing" of the MR sensing configuration. There is no physical contact and hence no magnetic exchange coupling active between the hard-magnetic film and any magnetic part of the MR sensor. Instead, the biasing flux is injected at the boundary between sensing and end regions as defined by a superpositioned conducting layer. The disadvantage of this invention is that magnetostatic control does not render the end-segments totally inactive. Thereby, substantial pick-up of extraneous magnetic signals occurs from the end-regions.
U.S. Pat. No. 4,589,041 describes a differential sensor which uses a pair of plane-parallel MR segments that mutually bias each other in the transverse direction as is shown in FIG. 2(b). Such a design is of particular interest as it offers substantially unlimited spatial resolution capability without the use of magnetic shields. However, the teachings of this invention do not include associated longitudinal biasing provisions and no prior art biasing scheme is readily applicable to a differential sensor design.
Thus, there is a need in the prior art for improvements in the longitudinal biasing provisions and there are no known biasing schemes that satisfy the requirements of differential sensor embodiments. The needed improvements relate to the simplicity of fabrication, the sensors-spatial resolution capability and its long-term reliability. It is the objective of this invention to provide these improvements.